With the continuing trend towards miniaturization of electrical systems such as information handling systems (computers), set top boxes, automotive computers and the like, the margin of error for connecting components in these systems continues to decrease. As the pitch of the array tightens, i.e. as the distances between balls decreases, the tolerances for misalignment get smaller and the risk of connections to incorrect pads gets bigger.
The trend of interconnection technology is towards connectors, referred to respectively as ball grid arrays (BGA) and column grid arrays (CGA), that use arrays of small balls or columns, typically of 90/10 solder, i.e. solder that is 90% tin and 10% lead. The balls or columns are attached to contact pads on the module with lower melting temperature solder, typically 63/37 solder (63% tin and 37% lead). The module, with balls or columns attached, is positioned so that the balls or columns contact pads or other conductors on the surface of another electrical component, such as a printed circuit board, a chip carrier that is connected to a printed circuit board, or another module. The pads on the other component will also have solder paste, usually 63/37 solder, screened on to accept the module. Typically, the assembly is then heated to melt the solder paste and bond the two components together. With this type of attachment, the module is located and held by the reflowed 63/37 solder.
The process of soldering a module to a PC Board or carrier stresses both components of the assembly. Post assembly requirements such as repairs, replacements and upgrades further stress both components, and other nearby components as well. In addition, soldering complicates field repairs, upgrades and other replacements. With solder connections, field replacements must usually be complete assemblies. Where this is not possible or desirable, factory repairs are required.
In some systems, typically testing apparatus, solder balls or columns are simply pressed against pads on another component or interconnect device. Similar unsoldered mechanical connections are made in land grid array or pad-on-pad connections. These unsoldered mechanical connections, with arrays of balls, columns, lands, pads or other contacts pressed against other contacts, are referred to herein as "contact array connections." The arrays of balls, columns, lands, pads or other contacts are referred to as "contact arrays" or "arrays of contacts."
U.S. Pat. No. 5,468,158 to Roebuck et al discloses a system for mechanically connecting integrated circuit chips or dies with peripheral array connectors to a burn-in test board so that the chips can be tested or burned in to produce "known good die." The chips are mounted on a substrate with conductive traces ending in bumps that contact pads on the bottom surface of the chip. Visual positioning equipment is used to place and align chip or die on the substrate. Optical alignment marks are provided on the substrate for that purpose.
The die is pressed against a substrate by a lid, which is pressed against the upper surface of the die (or against an intervening electrical biasing clip) by a spring. The spring is compressed by one of several mechanisms or systems that connect to two posts extending upwardly from the substrate. One of these mechanisms comprises a rotary latch assembly with two ramps having openings through which heads of the posts are inserted. As the latch assembly is rotated the heads of the post move up the ramps, compressing the spring and pressing the die against the substrate.
This entire subassembly is then inserted into a socket subassembly with pins that are inserted into holes in the burn-in test board. The socket assembly includes a cover with means to enlarge an opening for the substrate assembly and die, so that pads on the periphery of the thin film interconnect (substrate) can contact electrical connectors in the base of the socket.
The Roebuck system is bulky and complicated, and is primarily for laboratory/factory test applications. Most of the embodiments require tools for assembly. The ramps on the rotary latch version are subject to wear and binding because the heads of the mounting post make line contact with the ramps.
The two post mounting system and centrally located coil spring press against the center of the die. Forces are not evenly distributed. With manufacturing variations and other irregularities, the die may tip and wobble and fail to make good electrical contact with some of the contacts in the peripheral array.
The peripheral array limits the number of available contacts. It is not readily apparent how this system could be adapted to ball grid arrays or column grid arrays which, because of the much larger number of available contacts that can be provided within a given surface area, are becoming the connector of choice for the electronics industry.